This invention relates to joint prostheses, in particular finger joint prostheses and most particularly prostheses for the metacarpophalangeal joint. A variety of prostheses for use in the surgical repair of diseased or damaged finger joints are known. One type of prior art device (see, e.g., U.S. Pat. Nos. 3,462,765 and 3,593,342) consists of a unitary flexible rubber body having stems for fixation within the intramedullary canals of the two bones forming the joint. Although widely used, this type of prosthesis has certain disadvantages. For example, rubber with adequate flexibility has less than the desired level of mechanical strength, and some mechanical degradation and even breakage of this type of prosthesis under the conditions of actual use have been observed clinically. Also, these prostheses can have a highly unnatural feel in the body because they do not always flex at the desired point or adequately control radio-ulnar rotations, and because they provide an insufficient moment arm to the extensor tendon to overcome extensor lag.
A second type of prior art finger joint prosthesis (see, e.g., U.S. Pat. Nos. 3,506,982; 4,011,603 and 4,059,854) comprises two components, with the metal protuberant head portion of one component snapping into and retained by the plastic housing of the other component. This type of prosthesis generally provides a more accurate reproduction of the movement of the natural finger joints than the unitary rubber implants, but nevertheless is basically a linked device and thus can feel awkward and overly constrained in the body of a patient. The forces inherent in the mechanical constraint of this type of prosthesis are transmitted to the plastic housing, which can fail by creep or fracture.
An always potential problem with the use of joint prostheses generally is the loosening of the fixation between an intramedullary stem of the prosthesis and the wall of the intramedullary canal of the host bone under the influence of the stresses experienced in use at the bone-prosthesis fixation interface. This problem may develop whether the fixation is by cement, bone tissue ingrowth, or alternative method. Torsional, shear, tensile and compressive forces upon the prosthesis are generally transmitted to the bone-prosthesis fixation interfaces, which may then be weakened by the constantly recurring action of these forces.
Therefore, it would be highly beneficial to the art to provide a class of joint prosthesis wherein the transmission of forces acting upon the prosthesis to the bone-prosthesis fixation interfaces is minimized, thus assuring the integrity of the fixation obtained after implantation and, e.g. tissue ingrowth, and also reducing the risk of subsequent loosening of the fixation during use of the prosthesis. Also, in the case of the metacarpophalangeal joint, it would be highly beneficial to provide a new prosthesis reproducing the motion and degrees of freedom of the natural joint, without, of course, sacrificing the mechanical stability of the device.